Conventionally, as a power supply apparatus for general appliances for household use such as home electric appliances, a switching power supply apparatus has been widely used that includes a semiconductor device controlling an output voltage with use of a switching operation of a semiconductor (a switching element such as a transistor), for the purpose of improvement of power efficiency by reduction in power consumption.
Especially in recent years, there has been a demand for reduction in power consumption in electric appliances for prevention of global warming. Attention is particularly focused on power consumption in a standby mode of appliances having a standby function. There has been a great demand for a switching power supply apparatus having a lower power consumption in the standby mode.
Generally, in a light load state such as the standby mode or the like, a dominant energy loss in a switching power supply apparatus is a switching loss due to a switching operation. As one of known arts for improving the power efficiency in the light load state, power supply operation is performed under Pulse Frequency Modulation (PFM) control in which a switching frequency is decreased in accordance with a load current.
FIG. 17 shows a configuration example of a conventional switching power supply apparatus that includes a semiconductor device having a PFM control circuit.
As a load current output in a rated load state decreases, an output voltage increases. Regarding information indicating this increase of the output voltage, a feedback signal is input to an FB terminal via an output voltage detection circuit 5, and Pulse Width Modulation (PWM) control is performed such that a current flowing through a switching element 2 decreases in accordance with a value of a signal output from a feedback signal control circuit 11. A state under PWM control corresponds to a range A in FIG. 3, and a switching operation is performed at 100 kHz for example.
When the load further decreases, the switching power supply apparatus switches from PWM control to PFM control, and operates so as to vary a switching frequency of the switching element 2 in accordance with the load state. A state under PFM control corresponds to a range B in FIG. 3. PFM control is performed such that as the load decreases, a feedback current IFB increases, a voltage EAO decreases, and the switching frequency decreases. In this way, the switching power supply apparatus in a light load state reduces the number of switching operations by performing PFM control to reduce the sum of switching losses as much as possible, thereby to improve a power efficiency.
According to such a conventional switching power supply apparatus, however, when a load is constant, a switching frequency of a switching element operating a switching operation is fixed. This causes a problem that spectral components of a high-frequency current flowing through the switching element concentrate in the switching frequency and its harmonic components, and as a result noise (electrical noise) easily occurs. Such noise is called a high-frequency noise or a terminal noise. Components such as filter circuits against the noise are necessary, and this hinders size-reduction and cost-reduction of the switching power supply apparatus.
Here, the terminal noise represents a leakage voltage that is induced due to leakage of a switching frequency by a switching operation and harmonic components thereof from a commercial AC power supply to the outside. A magnitude of the terminal noise is expressed by indicators such as a peak value that is the maximum amplitude value of the noise, a quasi-peak (Qp) value that is close to the peak value and varies depending on an amplitude and a frequency of the noise, and an average value. When the switching frequency is constant, the peak value, the Qp value, and the average value are equal to each other with no variation. The standard value of the average value is set lower than the standard value of the Qp value. However, in the case where the Qp value and the average value are equal to each other as described above, the Qp value needs to be decreased to the average value.
Also, there is a case where when a load of a switching power supply apparatus operating under PFM control decreases, the switching power supply apparatus operates at an frequency in an audible frequency region of 20 kHz or less (hereinafter, referred to as audible region). In particular, when there is a small variation in load in a standby mode or the like, if a switching frequency is fixed at a particular frequency in the audible region, sound is sometimes generated from a transformer, a ceramic capacitor, and so on which are generally used in the switching power supply apparatus.
In response to this, a measure may be taken in which the minimum switching frequency under PFM control is set to greater than 20 kHz beyond the audible region. It is true that this measure prevents generation of sound in the transformer, the ceramic capacitor, and so on. Even in the light load state, however, the number of performance of switching increases due to the minimum switching frequency which is set high. This results in increase in switching loss to hinder improvement of power efficiency.
In addition to the above measure, there are known measures such as a measure of performing PFM control by skipping the audible region, and a measure of impregnating the transformer, the ceramic capacitor, and so on with a resin or the like. However, these measures cause increase in area of circuits, increase in cost, and so on. As a result, there arises a demand for improvement in the trade-off relationship between performance and cost.
In response to the demand for suppression of noise occurrence as described above, there have conventionally been proposed switching power supply apparatuses such as disclosed in Patent Literatures 1 and 2. The switching power supply apparatuses such as disclosed in Patent Literatures 1 and 2 can reduce an average value of a terminal noise by diffusing a switching frequency of a switching element within a predetermined frequency range.